jeudi 27 juin 2024

Support for Biden and Trump on the eve of the first debate



A short message on the polls of the 2024 US presidential election. I decide to follow the polls early on this time since we already know who are the candidates. This will allow me to better follow what is happening and the quality of the polls. I will introduce two graphs, one on the global trends in vote intention and a second one on the trends according to mode of administration and sampling sources combinations.


Trends in voting intentions since January 1st

 This first graph portrays the trends in the proportion of support for each main candidate on what is called the 2-party share, that is, the sum of voting intentions for Trump and Biden. This is the only comparable metric. It is important to note that pollsters do not always publish comparable estimates. Some include the small candidates, other don't. Some group together the small candidates and the "Don't know". Some ask the question twice, once with all the candidates and the second time with only Trump and Biden (we selected only the first question for analysis). Most pollsters do not exclude the "don't know", a proportion that differs substantially between pollsters. In short, it is essential to compare using the 2-party share.

 The graph illustrates what happened since January 1st.  As you can see, the gap between Trump and Biden seems to have decreased. Trump is still ahead but by a very slight margin. However, we see that some polls differ substantially from the others: Some place Trump way ahead, other put Biden ahead of Trump. Be reassured, these "outliers" do not substantially influence the trends presented here given the method used (local regression). However, is there a possible explanation?


Trends according to mode-source combinations

The following picture shows the same data for two types of mode-source combination. One type of combination is Web Opt-in panel only. The other type is Multiple modes, a combination that includes at least partially the use of random or quasi-random sampling. I have shown here that mode-source combinations had a substantial impact on estimates in the US 2020 election. The question is whether the impact is present from the beginning. It was not that evident until recently.

There are three types of mode-source combinations, described in the article referred to above. The third type is "one mode, random or quasi-random". It includes telephone polls. There are not enough polls in this category for analysis right now. Therefore, these polls are not included. The Activote polls are not included either because we cannot categorize them (see below).

The graph shows the trends in support for Biden portrayed by the two types of mode-source combination. As during the campaign in 2020, web opt-in polls seem, on average, not to detect any movement, besides a small linear increase in support for Biden, while polls using multiple modes do. They show an increase followed by a recent decrease. Who is right?



The recent decrease portrayed by polls using multiple modes may be partly due to some specific practices. For example, the "outlier" putting Biden under 45% recently is a Rasmussen Reports poll. I decided to check and realized that the question on voting intention followed 10 different questions on immigration policies and preferences, a topic that is quite polarising. This practice may have biased the estimate in favor of Trump. However, even if I drop this poll, the difference between the two combinations are still present. 

This being said, Rasmussen published its questionnaire, a practice not always present. Whitout the questionnaire, we can't know the rank of the question on vote intention and whether the questions asked before it may bias the estimates. My understanding is that the current practice is to ask about voting intention at the beginning of the questionnaire and not ask questions that are polarising before the vote intention question.

A second pollster used a methodology that is quite disturbing. Activote seems to rely only on people  recording their vote intention on a free application available on phones and computers. The application is always open so that people can answer when they want. Activote publishes estimates when they feel they have enough answers (200 for state polls). The polls are conducted over 10 to 15 days. Activote does not say whether they check for people answering multiple times. Their estimates are quite stable, around 48-49% for all polls.


Biden and Trump are close, very close, everybody knows. But we also have to be aware that the way the polls are conducted may have an impact on the estimates that are published. In addition, when looking at polls, it is important to check all the information that we may have -- or not -- on the methodology, including the questionnaire. As a general rule, the less information you have on a poll, the less trusty the poll is. But sometimes, the information provided also leads to doubt about the numbers published. However, it still gives you some relevant information.

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